Since a silica aerogel, as a high specific area, ultra-porous material having a porosity of about 90% to 99.9% and a pore diameter of about 1 nm to 100 nm, has excellent characteristics such as ultra lightweightness, ultra insulation, and ultra-low dielectric constant, research into the applications of the aerogel as a transparent insulator and an environmentally-friendly high-temperature insulator, an ultra-low dielectric thin film for a highly integrated device, a catalyst and a catalyst support, an electrode for a supercapacitor, and an electrode material for desalination as well as the development of an aerogel material has been actively conducted.
The biggest advantage of the silica aerogel is super-insulation having a thermal conductivity of 0.300 W/m·K or less which is lower than that of an organic insulation material such as a typical Styrofoam. Also, the aerogel may address fire vulnerability and generation of toxic gas in case of fire, i.e., fatal weaknesses of a typical organic insulation material.
In order to prevent structural collapse due to a shrinkage phenomenon occurred during drying, the silica aerogel is prepared by a method in which a hydrophobic silica aerogel is prepared and a surface modifier is then removed by pyrolysis.
Specifically, the silica aerogel is prepared by the steps of: preparing a silica sol by hydrolysis of tetra ethyl ortho silicate (TEOS) or water glass with an acid catalyst, adding a basic catalyst thereto, and performing a condensation reaction to prepare a hydrophilic wet gel (first step); aging the wet gel (second step); performing solvent substitution in which the aged wet gel is put in an organic solvent to substitute water present in the wet gel with an organic solvent (third step); preparing a hydrophobic wet gel by adding a surface modifier to the solvent-substituted wet gel and performing a modification reaction for a long period of time (fourth step); preparing a hydrophobic silica aerogel by washing and drying the hydrophobic wet gel (fifth step); and pyrolyzing the aerogel (sixth step).
Recently, in order to further extend applications of silica aerogel, a plan of improving mechanical properties in addition to original properties of the silica aerogel has been reviewed, and, for example, a metal oxide-silica composite aerogel, in which a metal oxide is introduced, is being developed.
In general, the metal oxide-silica composite aerogel is being prepared by the steps of: adding a metal ion solution and an acid catalyst to a water glass solution and performing a reaction to prepare a metal oxide-silica composite wet gel (step 1); and washing and drying the wet gel (step 2) (see FIG. 1). With respect to the above preparation method, in order to allow a network structure of the aerogel to be well-formed, the reaction proceeds under a low acidity (pH) condition by adding the acid catalyst. However, since the metal oxide is formed within a high pH range, the metal oxide in the prepared metal oxide-silica composite aerogel is contained at a low content ratio. Thus, since an effect of improvement in mechanical properties by the introduction of the metal oxide is insignificant, it has a limitation in industrial applications.
Thus, there is a need to develop a method which may prepare a metal oxide-silica composite aerogel containing the metal oxide at a high content ratio.